Speed Converter

Type a speed, choose the unit you are starting from and the one you want, and the converted value appears as you type. The four units here are the ones that actually collide in daily life: kilometers per hour on road signs across most of the world, miles per hour in the United States and United Kingdom, meters per second in science and weather reports, and knots at sea and in the air. Read the result as an exact conversion, because the factors it rests on are defined constants rather than rounded estimates.

How the conversion works

Every unit is stored as an exact factor relative to km/h, so a conversion is one multiply and one divide:

result = value × factor_from ÷ factor_to

With the defaults — 100 km/h into mph — that is 100 × 1 ÷ 1.609344 = 62.1371 mph. None of the factors are approximations. The statute mile has measured exactly 1,609.344 meters since the 1959 international agreement, the knot has equalled exactly 1.852 km/h since 1929, and the m/s factor of 3.6 is pure arithmetic: 3,600 seconds in an hour over 1,000 meters in a kilometer.

Routing every value through km/h keeps the arithmetic clean. Converting knots straight to mph gives the same answer as going knots to km/h to mph by hand, because the shared factor cancels and no rounding accumulates in between.

Where the metric units come from

A speed unit is a length divided by a duration, so it inherits the history of both. The metre began as a piece of the planet. In 1791 the French National Assembly defined it as one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. The astronomers Jean-Baptiste Delambre and Pierre Méchain spent years surveying the arc between Dunkirk and Barcelona, and in 1799 France cast the result into a platinum bar, the mètre des Archives. A platinum-iridium prototype replaced it in 1889, wavelengths of krypton-86 light took over in 1960, and since 1983 the metre has been fixed to the speed of light itself: the distance light travels in a vacuum in 1/299,792,458 of a second. The second has been tied since 1967 to the caesium-133 atom. A value in meters per second is therefore anchored to two of the most precise measurements people know how to make.

Kilometers per hour and meters per second are the same system at different scales. The factor of 3.6 between them is only the ratio of the prefixes, 1,000 meters against 3,600 seconds, which is why multiplying m/s by 3.6 lands exactly on km/h with nothing rounded away.

The mile and the knot

The mile is older than the metric system. It descends from the Roman mille passuum, the thousand paces of a marching legion, roughly 5,000 Roman feet. English usage drifted for centuries until an Act of Parliament under Elizabeth I in 1593 fixed the statute mile at eight furlongs, or 5,280 feet and 1,760 yards. The count of feet held after that, but the yard itself varied slightly from one country to the next, so in 1959 the United States, United Kingdom, Canada, Australia, New Zealand and South Africa signed the International Yard and Pound Agreement and defined the yard as exactly 0.9144 meters. That one line makes the mile exactly 1,609.344 meters and supplies the final digits of the mph factor in the table.

The knot comes from navigation rather than the road. A nautical mile was meant to equal one minute of latitude, a sixtieth of a degree, so a navigator could step distance straight off the latitude scale printed on a chart. Because the Earth is not a perfect sphere, that minute changes slightly with position, and in 1929 the First International Extraordinary Hydrographic Conference in Monaco settled on a single international nautical mile of exactly 1,852 meters. A knot is one nautical mile per hour, which is why it equals 1.852 km/h to the millimeter. The United States kept a slightly different national value until 1954 and the United Kingdom until 1970.

Measuring speed with a rope

The name knot is literal. Sailors judged a ship's speed with a chip log, a weighted wooden panel tied to a long line and thrown over the stern to drag in the water while the line ran out; the first known printed description of the device is in William Bourne's A Regiment for the Sea of 1574. The line carried knots tied at regular spacing — the figure that came to be accepted was 47 feet 3 inches, or 14.40 meters — and one sailor counted how many slipped through his fingers while another timed a sand glass, half a minute in Bourne's early practice and 28 seconds in the reckoning that stuck. The number of knots was, directly, the speed in nautical miles per hour. The spacing was chosen so the arithmetic came out right: 47 feet 3 inches in 28 seconds is 20.25 inches per second, or 1.85166 km/h, within 0.02 percent of the value defined in 1929, which is a remarkable result for a rope and an hourglass.

Cars met the same problem over three centuries later and answered it mechanically. The Croatian engineer Josip Belušić patented an electric speedometer in 1888 and showed it at the 1889 Exposition Universelle in Paris, and the German engineer Otto Schulze of Strasbourg filed the eddy-current design that motoring settled on with the patent office in Berlin on 7 October 1902. A cable driven off the transmission spins a permanent magnet inside an aluminium cup; the eddy currents it induces drag the cup around against a hairspring, so the needle settles at a deflection proportional to speed, with no rope and no timing at all.

Why forecasts and road signs disagree

Each national weather service reports wind in whatever unit its audience already reads. The United States uses mph, Canada and Australia use km/h, the Nordic countries and Russia use m/s, and most marine and aviation forecasts use knots. Meters per second is the unit the World Meteorological Organization recommends for reporting wind, and since 2010 the International Civil Aviation Organization has recommended it as well. The same 15 m/s wind can appear as 54 km/h, 33.6 mph or 29.2 knots depending on the source, and on the Beaufort scale it is force 7, a near gale. A useful anchor for the SI unit: 10 m/s is 36 km/h, the top of force 5, a fresh breeze, where small trees in leaf begin to sway. Umbrellas only become hard to hold at force 6, from 10.8 m/s up.

Road signage explains most of the surviving mph holdouts. Australia switched its signs to km/h on 1 July 1974 and Canada over the Labour Day weekend of 1977, while the United States and United Kingdom kept miles, so drivers crossing the US–Canada border still run this conversion every day. Aviation kept knots almost everywhere, even in fully metric countries, because charts and air-traffic instructions were built around the nautical mile long before the ground below went metric. The METAR code that carries airfield weather still allows meters per second for the states that prefer it.

Speed is running pace turned upside down

Runners think in pace — minutes per kilometer or per mile — which is the inverse of speed rather than a unit of it. 12 km/h corresponds to 5:00 min/km, and easing to 6:00 min/km means 10 km/h. Because that conversion divides into 60 instead of multiplying by a fixed factor, it lives in its own tool: the running pace calculator handles min/km, min/mi and race-time predictions.

Frequently asked questions

How fast is 100 km/h in mph?

100 km/h is 62.1371 mph, because a mile is exactly 1.609344 kilometers. Going the other way, 100 mph is 160.93 km/h. Speed limits line up loosely across borders: the UK motorway limit of 70 mph works out to 112.65 km/h, close to the 110 or 120 km/h limits common across Europe.

What is a knot and how fast is it?

A knot is one nautical mile per hour, which is exactly 1.852 km/h or about 1.1508 mph. The name comes from the knotted rope sailors paid out behind a ship to gauge its speed. A container ship cruising at 20 knots is doing 37.04 km/h, and an airliner at 480 knots is doing roughly 889 km/h.

How do I convert m/s to km/h without a calculator?

Multiply by 3.6, since an hour has 3,600 seconds and a kilometer has 1,000 meters. So 10 m/s is 36 km/h and 25 m/s is 90 km/h. For m/s to mph, multiplying by 2.2 lands within about 2 percent of the exact answer.

Why do weather forecasts use different wind speed units?

Each national weather service picked the unit its audience already knew. The US reports wind in mph, Canada and Australia use km/h, much of continental Europe uses m/s, and marine and aviation forecasts everywhere use knots. The same 15 m/s wind can appear as 54 km/h, 33.6 mph or 29.2 kn depending on the source.